It is simple to say that the physical properties of an individual transistor within an integrated circuit tend to determine the performance of the transistor. However, understanding how these properties—the number of which is virtually infinite—affect the performance of the transistor is another matter altogether. It is an even greater subsequent step to then predict how certain properties will be affected by various processing conditions, and how variations in these effects can be compensated for.
As the term is used herein, “integrated circuit” includes devices such as those formed on monolithic semiconducting substrates, such as those formed of group IV materials like silicon or germanium, or group III-V compounds like gallium arsenide, or mixtures of such materials. The term includes all types of devices formed, such as memory and logic, and all designs of such devices, such as MOS and bipolar. The term also comprehends applications such as flat panel displays, solar cells, and charge coupled devices.
Integrated circuits are formed of many layers of different materials, which layers are patterned so as to form desired structures that interact with one another according to predetermined designs. Thus, it is of vital importance that many of these layers be formed to very exacting tolerances, such as in their shape, thickness, and composition. If the various structures so formed during the integrated circuit fabrication process are not precisely formed, then the integrated circuit tends to not function in the intended manner, and may not function at all.
The variations of many important integrated circuit properties are dependant upon many factors that are well known. However, some variations occur in a manner that is seemingly not predictable.
What is needed, therefore, is a better understanding of how to compensate for variations in integrated circuit properties.